1. Field of the Invention
The present invention relates to a solid-state image pick-up device and a method of manufacturing the solid-state image pick-up device, and more particularly to a solid-state image pick-up device having a structure in which a gate oxide film includes a silicon nitride film and a method of manufacturing the solid-state image pick-up device.
2. Description of the Related Art
In a solid-state image pick-up device in related arts, as shown in an example of a sectional structure in FIG. 7, a photodiode section and a charge transfer section are formed in a p well 11 provided on the surface of a semiconductor substrate 10, and electric charges generated in the photodiode section are led to a transfer channel comprising an n-type impurity region 14 by applying a voltage to the charge transfer electrode of the charge transfer section, and are sequentially read. In the charge transfer section, the electric charges generated in the photodiode section are led to the transfer channel comprising the n-type impurity region 14, and a gate electrode 18 to be a charge transfer electrode and reading electrode (hereinafter referred to as a reading electrode) is formed through a gate oxide film having a three-layer structure including a silicon oxide film 15, a silicon nitride film 16 and a silicon oxide film 17 on the n-type impurity region 14.
Thus, the gate oxide film provided under the reading electrode of the solid-state image pick-up device has a so-called ONO structure in which a silicon nitride film to be a gate having a high breakdown voltage is interposed between silicon oxide films. With this structure, a thin gate oxide film having a high breakdown voltage is essential in the solid-state image pick-up device which is thin and is more microfabricated recently. The ONO structure is essential to a reduction in the thickness of a gate film.
In the solid-state image pick-up device having such a structure, when a light is incident on a pixel section, it is photoelectrically converted in an n-type impurity region 12 so that a signal charge a is generated and is moved to the transfer channel 14 when a reading pulse is applied to the gate electrode 18 to be the charge transfer electrode and reading electrode. On the other hand, a signal charge by generated in the vicinity of the surface of the substrate is accelerated by an electric field through the reading pulse, and a part thereof is changed into a hot electron and is trapped into the silicon nitride film, thereby causing the aging of a reading gate voltage.
With the advance of the microfabrication of the device, the impurity concentration of the n-type impurity region 12 tends to be increased due to a reduction in a resistance so that the convergence of an electric field is more increased on the end of the reading electrode. Moreover, since a gate length is reduced, the number of collisions of an electron is decreased so that the frequency of the generation of the hot electron tends to be increased. Consequently, the aging of a voltage to be applied to the reading gate has become a serious problem.
In the such a solid-state image pick-up device, thus, there is a problem in that a gate structure which has a high breakdown voltage and can have a thickness reduced and a gate structure in which aging is not generated by a hot electron have a trade-off relationship and both of them cannot be satisfied at the same time.